Apple Tree Canker - Advice/Opinion?

3 years ago I bought 2 apple trees (Bramley and Herefordshire Russet) and have since trained them as espaliers - both now growing their third layers. Both trees have done well and blossom/fruit set has been good this year. However the Russet has a large area of canker 'damage' on the main trunk just above the first layer. It affects about half the circumference. The growth above looks healthy - I have googled the problem and am relieved to read that this will not necessarily kill the tree. However should I cut the damage out of the tree, ie above the first layering, or leave it and see how the tree copes with it? If I do cut out the canker, will the tree re-grow so that I can continue with the espaliering.

I seem to have the same problem on my small apple tree(name unknown), only the canker damage is in the middle of the trunk of the apple tree below the first branch; as the apples themselves seem to be ok does this mean I will have to destroy the tree?